Preparing a deck for a modified bitumen roof involves several key steps to create a smooth, secure surface that supports the roofing material’s durability. The deck should be made from plywood or oriented strand board (OSB) sheathing rated for roof applications, typically at least 5/8-inch thick to provide strength and minimize flexing. It's important to ensure the deck is flat and free of any bumps, depressions, or gaps larger than 1/4 inch, as imperfections can cause the membrane to blister or wear prematurely. Use a straightedge or level to check flatness, sanding down any high spots and filling low spots with a high-quality roof patch. Fasteners should be corrosion-resistant screws or nails designed for roof decking, spaced according to manufacturer guidelines—generally every 6 inches along edges and 12 inches in the field. Overdriving fasteners can damage the sheathing, so they should be flush with or just below the surface.
A safe DIY check before installing the modified bitumen is to walk the deck while inspecting it for soft spots, loose boards, or any signs of water damage or rot that may require repair or replacement. If the deck has existing layers of old roofing material, they should be fully removed unless the roofer confirms otherwise, as residual materials can affect adhesion. It's wise to call a professional roofer if you notice significant unevenness that can’t be easily fixed, signs of structural damage, or if you’re unsure about fastener placement and deck quality. When speaking with your contractor, helpful questions include: “What type of sheathing and fasteners do you recommend for my roof deck, and how will you ensure it’s flat enough for the modified bitumen?” and “How will you inspect and address any hidden damage before installation to maintain my roof warranty?” These queries can help ensure your deck prep supports a long-lasting membrane.